Ball mills and attritors have been used for many years to produce fine powders. In ball mills, the energy input to the powder charge is provided by the rotation of the mill, a cylindrical cell or vial about a horizontal axis, so that hard balls within the mill are tumbled with, or onto, the charge in the mill. In attritors, metal arms are used to stir the ball charge.
As noted by Y S Benjamin in his article in Scientific American, volume 234, page 40 (1976), it was appreciated in the early 1970's that in addition to creating powders, ball milling could be used to produce solid state reactions which result in the synthesis of new alloys from elemental powders. It was also discovered that ball milling can modify an alloy structure. The first of these techniques (the synthesis of alloys) is known as "mechanical alloying"; the second technique has been termed "mechanical grinding".
When mechanical alloying is used to produce new materials, there is a combination of repeated welding, fracturing and rewelding of a mixture of powder particles having a fine microstructure together with a rapid interdiffusion process.
Both mechanical alloying and mechanical grinding have been effected using either the vibrating milling technique or the rotating milling technique. In vibrating-frame mills, hardened steel balls are caused to impact substantially vertically upon the powder charge. Local overheating of the particles can occur as a consequence of the mill structure. This local overheating is difficult to remove. In addition, the mixing of the particulates is very slow (and in some designs of mill, is almost non-existent). Thus rotating mills, in which the steel balls roll along a circular arc on the inner wall of the mill chamber or vial, are preferred for mechanical alloying.
In rotating mills, the powder charge is spread on the inner surface of the chamber. This ensures that heat generated within the chamber is removed by conduction through the cylindrical wall of the chamber and that there is effective mixing of the powder constituents. However, when using rotating mills, it is not possible to provide the impact energy of the balls that is achieved in vibrating-frame mills when a rotating ball mill is used.